PROJECT SUMMARY/ABSTRACT This proposal describes a five-year mentored training program for the career development of a physician- scientist to examine how cellular cytotoxicity mediates host defense to intracellular pathogens in human skin. To study this the disease leprosy, caused by intracellular infection with M. leprae, will be utilized as a model. Currently, it is thought that the antimicrobial immune response in resistant T-lep vs. susceptible L-lep is conferred in large part by cytotoxic T lymphocytes (CTL). CTLs, however, are heterogeneous such that the key T cell population that contributes to host defense is unknown. Here we propose to study a newly identified population of CTLs which co-express granzyme B (GZMB), perforin (PRF) and granulysin (GNLY), termed `tricytotoxic' (T-CTL) and posit that this CTL population contributes to host defense against M. leprae. Using high throughput RNA sequencing coupled with functional studies this proposal will investigate the breadth and mechanism of the cytotoxic T cell response to an infectious agent in human skin including: Aim 1) Determine molecular markers that define the T-CTL subset, and ascertain the functional relevance of T-CTL in host defense at the site of disease, and Aim 2) Identify the mechanisms that differentiate, expand, and functionally regulate T-CTL. This approach directly addresses several goals of NIAMS as outlined in the ?Immunobiology and Immune Diseases of Skin Program? including regulation of skin immune responses by T cells and the characterization of the soluble mediators they express including antimicrobial peptides in a human skin disease. The candidate previously completed a PhD studying how adaptive immunity may be shaped to mutating pathogens and has completed clinical residency in dermatology, clinical fellowship in dermatopathology as well as post-doctoral fellowship training through the STAR program research track at UCLA. This proposal will afford him the ability to develop an understanding of a completely human system for research permitting the expansion of clinical translational skills due to the large component of leprosy patients and healthy controls. Additionally he will develop new molecular techniques including high throughput RNA sequencing and the skill set required to analyze such data. Outcomes from this research along with the newly developed skill sets will be widely applicable to the study of any skin disorder. This critical mentored phase of training will be guided by Robert Modlin, MD, an expert in the fields of leprosy, immunology, and translational cutaneous medicine who has successfully trained numerous independent investigators. In total, this research will have far reaching implications for understanding how the immune system in general protects against infections, providing new avenues for exploration towards to the goal of immune based therapies. This program will allow the candidate to develop the skills and tools needed to embark upon this research project, while having the necessary mentorship and support needed towards the goal of maturing into an independent investigator.